ease up vs slacken off

ease up

verb
  • To become more relaxed or less demanding. 

  • To reduce one's speed. 

slacken off

verb
  • To slacken or lessen; to become less intense. 

  • To do (something) less intensely. 

How often have the words ease up and slacken off occurred in a corpus of books? (source: Google Ngram Viewer )